I predicted it, and I mean it. Orange liqueurs deserve recognition for what they are: an essential ingredient in some of the world's greatest cocktails. The Sidecar, Margarita, Mai Tai, and Cosmopolitan all have orange liqueur in them. Leaving your fate to chance—in other words, not asking for your choice of orange liqueurs—is a little like walking into a grocery store and asking for beef. When the butcher asks you what cut, you say, "Oh, whatever."
Before I point you to some great resources for researching orange liqueurs, let's try and make some sense of the category itself. Drinks historian David Wondrich wrote on eGullet, "Unfortunately, the history of Curaçao is a sort of third rail for the would-be drink historian; I've found it so, anyway—as soon as you think you've got something figured out, something else comes up to prove you wrong."
Wondrich then goes on to describe two broad categories of orange liqueurs: white Curaçao and orange Curaçao. Where's triple sec? Well, triple sec is a phrase of virtually unknown origin. Does it refer to a style of liqueur or the third recipe from Cointreau? This is much debated. But for our purposes, it's enough to separate the dark stuff from the lighter stuff. Besides, Cointreau dropped the "triple sec" moniker a long time ago and Curaçao came first.
So what is Curaçao? Easiest answer: an island off the coast of Venezuela. Is the liqueur Curaçao always made in Curaçao? Well, not exactly. For instance, Grand Marnier used to be known as Curaçao-Marnier. I think the underlying commonality is the use of bitter orange peels.
Spanish settlers brought Valencia oranges to Curaçao. They didn't fare well and became puny, bitter cousins of the proud, sweet Valencia oranges. Some enterprising fellow decided to put the skins in spirits, et voila, they were fragrant and beautiful when macerated in alcohol. However, this does not mean that Curaçao necessarily originated in Curaçao. So forget spurious claims to being the original.
Well, here's your homework. Oh Gosh!, a cocktail blog, has the definitive blogger resource on orange liqueurs in its Orange Liqueur Showdown series. After a spin through the tasting notes, you'll be calling for the best and not asking for any old cut of beef.